Another option would be to create an encrypted disk image, and keep your photos library in that.
You would then have to mount the disk image and put in the password before you could open the photos app. Bear in mind though that if you ever forgot your password for the encrypted drive, the photos would be lost forever (unless backed up somewhere else). No recovery would be possible by anyone.
Finally the app "applocker" allows you to password protect individual apps. As I understand it the free version allows you to password protect one app - so if you only need it for photos app, that might work for you. This would NOT stop someone taking a copy of your photos library and opening it on a different mac, so if someone were determined to see your photos they still could. However, it would prevent casual browsing of your photos.